Electronic devices are becoming increasingly important in everyday modern life. Many electronic devices use pulse width modulation for a variety of purposes. In general, pulse width modulation is a technique where a continuous or analog signal is represented by a digital signal in which the duty cycle of the digital signal, over some period of time, varies directly with the value of the original analog signal. Duty cycle is the ratio of two numbers: the time in one period that signal is at logic high (or low) divided by the total period. This is usually represented as a percentage. The resulting digital signal is said to be Pulse Width Modulated. This signal can be converted back into analog form by the simple expedient of passing it through a low analog filter.
Pulse width modulation is used for a variety of purposes. For example, pulse width modulation is commonly used in measurement, control, power supplies and communication devices. As a specific example, pulse width modulation can be used to digitally encode sound. As another specific example, pulse width modulation is commonly used for motor control. Of course, these are just some of the many applications in which pulse width modulation is used.
One issue with pulse width modulation is the resolution of the modulation. In general, the resolution of a pulse width modulator is defined as how precisely the modulator can control the duty cycle of the signal. Resolution and bandwidth are competing criteria in many digital pulse width modulator circuits. Increasing resolution will decrease the bandwidth of the signal which can be represented by the pulse width modulated signal. For example, one typical pulse width modulator may be able to control the timing of high and low edge transitions on the pulse width modulated waveform used to compute duty cycle to within 20 nanoseconds, while other pulse width modulators may provide higher or lower resolution.
In some digital pulse width modulators the resolution of the modulator is tied to the clock frequencies used to generate the pulse width modulated signals. Unfortunately, in many cases it may not be possible or desirable to include a high speed clock for pulse width modulation. In these cases traditional techniques for digital pulse width modulation may be unable to achieve the desired resolution. Other solutions that can provide higher resolution suffer from other limitations, such as unacceptable difficulty in adapting these solutions to digital control
Thus, there is a continuing need for pulse width modulation techniques that provide relatively high resolution without requiring high speed clocks to achieve the needed level of resolution.